Vibrational spectra, co-operative intramolecular hydrogen bonding and conformations of calix[4]arene and thiacalix[4]arene molecules and their para-tert-butyl derivatives

摘要

The IR and Raman spectra and conformations of calix[4] arene, thiacalix[4] arene and their p-tert-butyl derivatives have been analysed within the framework of scaled quantum mechanics (SQM). It is shown that the introduction of four p-tert-Bu groups into the calixarene molecules influences the relative energies of their conformers and the enthalpy of the cooperative intramolecular H-bonding (Delta H-intra) almost negligibly. Delta H-intra, evaluated from Iogansen's rule, amounts to similar to 26-28 kcal mol(-1) for the calixarenes and similar to 20-21 kcal mol-1 for the thiacalixarenes, which essentially exceeds the enthalpies of non-cooperative H-bonds formed by related phenols. As a result of this strong bonding, bands of stretching, bending and torsion vibrations of an eight-membered cyclic system (OH center dot center dot center dot )(4) arise, e. g., two delta(OH)(4) bands are observed in the IR spectra of the most highly symmetric C-4 cone conformations of calix[4]arene and thiacalix[4]arene. The "duplication" of the number of OH infrared bands is a good new indicator of cooperativity of intramolecular H-bonding of the calixarenes.